In the course of progressive environmental legislation, the demand is growing for sensors that allow even the smallest quantities of pollutants may be reliably determined. In this context, above all, gas sensors play a great role which make possible the determination of gaseous pollutants in the ppm range, independent of the temperature of the measuring gas. The measuring signals of the gas sensor proportional to the quantity of pollutant are, in this connection, often so small that great measuring inaccuracy may not be avoided. A possible way out of this dilemma is represented by an indirect determination of pollutants.
Thus, a gas sensor disclosed in European Patent Application No. 241 751 monitors the content of ammonia, carbon monoxide, hydrocarbons, nitrogen oxides or sulfur dioxide in gas mixtures, but not the oxygen content. To be able to determine nitrogen oxides, among other things, a measuring method is proposed in which a known quantity of ammonia is added to a gas mixture as reaction gas which reacts with the nitrogen oxides at a catalyst of the gas sensor. If the quantity of ammonia originally added is known, one may conclude what the NOx concentration in the gas mixture is, by determining the remaining content of ammonia. The disadvantage of this method is that a device for adding the ammonia has to be provided.
It is the object of the present invention to make available a gas sensor which makes possible measuring various gas components in a gas mixture, reliably and using accurate timing.